warmer packets - how do they work?

[pakosouthpark]

I was looking at rain covers for cameras on the B&H website -
http://www.bhphotovideo.com/c/product/726806-REG/Camera_Duck_CDWS_SLRS_Standard_All_Weather_Cover.html

and they mention that they need warmer packs to work properly - anyone knows how this works? do you have any experience with them?

http://www.bhphotovideo.com/c/product/726888-REG/Camera_Duck_CDSW_W4_Large_Heat_Warmer_Packet.html

thanks

[TexPhoto]

Some hand warmers contain cellulose, iron, water, activated carbon (to speed up reaction), vermiculite (water reservoir) and salt (catalyst) and produce heat from the exothermic oxidation of iron when exposed to air. This type of heatpads normally lasts from 1 to 10 hours.

I have only used them to keep my hands warm. Works great.

[pakosouthpark]

what sort of temperature would they be?

[Mt Spokane Photography]

http://en.wikipedia.org/wiki/Hand_warmer